The system of 55 Cancri has five detected planets that span a broad range of masses, from about 10 times Earth to 4 or 5 times Jupiter. All travel on approximately circular orbits, like planets in the Solar System. The host is a Sun-like G8 star located at a distance of 12.53 parsecs (41 light years) in the constellation Cancer. It is cooler and dimmer than our Sun, with a bolometric luminosity only 60% Solar (all values Fischer et al. 2008).

For the primary star, Takeda and colleagues find a mass of 0.96 MSOL and an age of 5 billion years (Takeda et al. 2007). This age estimate is supported by the star's leisurely rotation period of 39 days (Fischer et al. 2008). Its planets have likely reached an evolutionary stage similar to our Solar System, whose age is estimated at 4.6 billion years.

55 Cancri is actually a binary system consisting of the yellow star described above (55 Cancri A) and a small red dwarf companion (55 Cancri B). With a spectral type of M4 and mass of 0.26 MSOL, the secondary star orbits the primary at a semimajor axis of about 1000 AU (Desidera & Barbieri 2006). This wide separation suggests that neither star would substantially inhibit the evolution of planets around the other. In fact, 55 Cancri B is a potential exoplanet host in its own right. Future radial velocity searches will establish whether it harbors its own planetary system.

55 Cancri A contains an unusually high proportion of heavy elements, with a metallicity measured at +0.315. Its M dwarf companion may be similarly enriched. Stellar enhancement in metals often coincides with the presence of giant planets at small semimajor axes and, less frequently, with the formation of multiple-planet systems (Fischer & Valenti 2005, Greaves et al. 2007). These generalizations are borne out by the configuration of 55 Cancri A’s five planets.

system architecture

Like the Solar System, the 55 Cancri system has clearly demarcated inner and outer regions, as shown in the diagram below (see also the comparative diagram of multi-planet systems). The boundary between them is drawn by the system’s ice line, which for a star of 55 Cancri’s mass is located at about 2.5 AU.

Architecture of the 55 Cancri system. Colored circles indicate the relative sizes of the 5 planets, assuming the actual masses provided by Fischer et al. 2008, the mass-radius relationships provided by Fortney et al. 2007, and substantial rock/metal cores. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale. White dots mark the ice line.

The inner system contains a cluster of four planets, all orbiting within a radius of 0.8 AU. This configuration is dominated by its most massive member, the Jupiter-size second planet (b). It includes a Super Earth or “Hot Uranus” of about 10 MEA (e) orbiting in a period of less than 3 days, as well as two additional objects that fall between the ice giants and the gas giants in mass (c, f). The orbits of the second and third planets (b, c) narrowly avoid a 3:1 mean motion resonance.

The three innermost planets (e, b, c) bear a strong family resemblance to the system of Gliese 876. In each case, observations reveal a star-hugging ensemble of planets comprising two gas giants (or quasi-gas giants) orbiting in or near a mean motion resonance, plus a smaller planet on an interior orbit (see Crowded Orbits). In each case, the inner planet is a tidally circularized Super Earth or Hot Uranus that has evidently been shepherded into its present position by the inward migration of the more massive planets (Ida & Lin 2005, Fogg & Nelson 2005, Mandell et al. 2007, Raymond et al. 2008a).

In 55 Cancri’s outer system, a single enormous gas planet (d) has so far been detected. It traces a wide orbit whose semimajor axis is almost 6 AU and whose period is more than 14 Earth years, even longer than Jupiter’s period. With a minimum mass around 4 MJUP and a relatively circular orbit, this planet must have exercised strong constraints on the system’s evolution, just as Jupiter did in our system.

five planets

55 Cancri is unusual insofar as the orbital inclination of its most massive planet can be estimated by photometric observations, yielding a value of about 53 degrees (Rivera et al. 2005, Fischer et al. 2008). If we make the reasonable assumption that the remaining planets have similar inclinations, then we can calculate the actual mass as well as the minimum mass for each one. (All system parameters cited below follow Fischer et al. 2008, except as indicated.)

1. The inner planet, designated 55 Cancri e, has a minimum mass of about 10.8 MEA, corresponding to an actual mass of 13.5 MEA (Fischer et al. 2008). Its orbit has an eccentricity approaching zero, a semimajor axis of 0.038 AU, and a period of 2.8 days. On the basis of mass, this planet can be classified as a Hot Neptune, implying the presence of a hydrogen atmosphere amounting to 10%-20% of its bulk composition.
   
   The best-understood Hot Neptune is GJ 436 b, for which a composition of 45%-70% rock/metal and 15%-40% ice has been calculated (Figueira et al. 2009). 55 Cancri e may contain an even higher proportion of rock and metal, given its likely origin through gas giant shepherding. According to Sean Raymond and colleagues, planets formed by shepherding will be composed of “a mixture of material that originated interior to the giant planet’s orbit” (Raymond et al. 2008a). As we know from the star’s metallicity, the protoplanetary disk surrounding 55 Cancri was heavily endowed with refractory elements. Thus any planet that formed inside its ice line will be metal-rich and perhaps volatile-poor.
   
   In fact, recent measurements of polarized light originating in 55 Cancri indicate a small radius for planet e, consistent with the radius estimate for GJ 436 b (Lucas et al. 2009) and thus with a bulk composition dominated by rock and metal.
   
   
2. The second planet, 55 Cancri b, has a semimajor axis of 0.11 AU (just outside the nominal range for Hot Jupiters) and a minimum mass of 0.84 MJUP (actual mass about 1.03 MJUP). Its period of 14.65 days places it very close to a 3:1 mean motion resonance with the next planet outward. Its atmosphere must be too hot to permit substantial cloud formation (Sudarsky et al. 2003). Planet b is plausibly interpreted as a smooth, deep blue sphere. Similar in mass and diameter to Jupiter, this planet has dominated the evolution of the inner system. It must have accreted outside the ice line at 2.5 AU and then spiraled through the primordial nebula to its present orbit, shepherding rocky material into ever-smaller semimajor axes and stirring up icy planetesimals in its wake (Mandell et al. 2007).
   
   
3. The third planet, 55 Cancri c, is a smaller, sub-Saturn world of about 0.17 MJUP (actual mass 0.21 MJUP = 67 MEA) with a semimajor axis of 0.24 AU. Its period of 44.4 days is almost triple that of its inner neighbor, but still only half that of the planet Mercury in our Solar System. This third planet is also likely to be a clear blue world, possibly similar in appearance to Neptune. Its mass places it near the uncertain boundary between ice giants and gas giants.
   
   
4. The fourth planet, 55 Cancri f, is smaller still, with a minimum mass of 0.14 MJUP (actual mass 0.18 MJUP = 57 MEA) and a semimajor axis of 0.78 AU. It also straddles the boundary between ice giants and gas giants, as it is far heavier than a confirmed ice planet like Neptune or GJ 436 b but lighter than a confirmed gas planet like Saturn. Since its primary star is cooler than our Sun, planet f occupies the system's classical habitable zone, despite an orbital radius similar to that of Venus. This planet is therefore likely to be a "water giant," with extensive white clouds of water vapor or ice crystals. Given its separation from the host star, it probably sustains a fairly rapid rotation, which would create dynamic weather patterns throughout its dense hydrogen-helium atmosphere.
   
   Planet f may be far enough from its central star to support a family of moons (see Potential Exomoons). Given the approximate mass scaling of 1:10,000 proposed by Canup & Ward (2006), any co-formed moons of this planet would be quite small, of Lunar mass or less. However, we cannot yet rule out the possibility that planet f has captured a rocky moon whose mass follows the 1:100 scale that characterizes the Earth-Luna system. In such a scenario, the hypothetical captured moon might be even more massive than Mars. Given our present state of knowledge, however, this appealing outcome seems unlikely.
   
   
5. With a semimajor axis wider than Jupiter's, the fifth planet of 55 Cancri could be accurately characterized only after the physical and orbital elements of the inner planets were resolved (Fischer et al. 2008). Its minimum mass is now estimated at 3.92 MJUP (actual mass 4.9 MJUP); its semimajor axis is 5.84 AU, with an orbital eccentricity of 0.06; and its period is almost 15 years.
   
   55 Cancri d seems more likely than most known extrasolar planets to resemble Jupiter and Saturn, especially because it may possess colorful cloud bands and even a system of rings. Unlike the system's four inner planets, the fifth planet is also likely to maintain a super-Jovian family of moons, with satellites potentially as massive as Mars. Unfortunately, such moons are likely to be cold and barren unless gravitational stresses induce substantial heating through volcanism.

evolution and migration

The architecture of 55 Cancri’s inner system is a product of migration. Planet b probably assembled near the system’s ice line and then spiraled into its present orbit through Type II migration, a form of interaction with the primordial gas disk that is unique to gas giants. In the process, planet b would have shepherded rocky material inside its shrinking orbital radius, causing collisions, accretions, and ejections that most likely led to the formation of the low-mass planet e (Fogg & Nelson 2005, Mandell et al. 2007).

Planet b’s passage through the nebula may also have triggered the formation of planets c and f. While it is conceivable that these two planets formed in situ (Fischer et al. 2008), it seems more likely that planet c, at least, assembled at a larger distance and then migrated into its present orbit. Given this planet's borderline mass, its passage through the nebula might have involved either Type II migration (as with planet b) or Type I migration, another form of interaction with the primordial gas that affects planets of Earth to Neptune mass. Since both kinds of migration tend to produce "a chain of resonant or near-resonant planets" (Raymond et al. 2008b), we may perceive the traces of such a history in planet c's current circular orbit just outside the 3:1 resonance with planet b.

Similar uncertainties of origin apply to planet f, another borderline case. This object may have formed in situ or attained its orbit through Type I or Type II migration. For planet f, however, we find no obvious traces of orbital shrinkage.

Whether any kind of migration or scattering has occurred in 55 Cancri’s outer system is unknown. Our own Solar System presents the case of four giant planets of modest mass, following well-separated orbits of extremely low eccentricity, between 5 and 30 AU. Despite the comfortably spaced orbits that we now observe, however, recent studies agree that our outer system had a violent past. About four billion years ago it was the scene of forced outward migration, planet-planet scattering, and perhaps even the ejection of an ice giant resembling Neptune (Tsiganis et al. 2005, Gomes et al. 2005, Desch 2007, Thommes et al. 2008).

Yet the only object in the Solar System available for detection by a team of planet hunters at the distance of the 55 Cancri system (using early 21st century technology) would be Jupiter, placidly circling our Sun at 5.2 AU. The gas giant 55 Cancri d presents an equally placid appearance that may hide an equally violent history.

additional companions

A gap wider than 4 AU separates the fourth and fifth planets. Recent analytical and simulation studies of the system’s orbital dynamics agree that this gap may harbor one to three stable orbits (depending on planet mass) and/or an asteroid belt (Fischer et al. 2008, Raymond et al. 2008a, Ji et al. 2009). Available methods are not yet sensitive enough to detect such objects, if in fact they exist. Raymond and colleagues found that a single Saturn-mass gas giant (~100 MEA) could achieve a range of stable orbits between 0.9 AU and 3.8 AU. Alternatively, up to three smaller objects resembling the known planet f (~50 MEA) might travel in regularly spaced orbits in the same region. Two-planet configurations were likelier than three, with typical stable outcomes placing one planet at 1.3-1.6 AU and the other at 2.2-3.3 AU. Three-planet solutions were still possible, with stable orbits at 1.1-1.2 AU, 1.6-1.9 AU, and 2.5-2.9 AU (Raymond et al. 2008a).

Regions of stability for terrestrial-mass planets are even more generous. Ji and colleagues found that Earth and Mars could fit neatly into the gap around 55 Cancri, without disrupting either their own orbits or those of the native quintet. So could similar telluric planets on wider orbits within 2.3 AU, which Ji’s group optimistically defined as the outer boundary of the system’s habitable zone (Ji et al. 2009). A more pessimistic limit would be 1 to 1.5 AU, given the star's low luminosity. Assuming the narrower boundary, a Cancerian version of Earth (with identical mass and semimajor axis) would become a snowball planet rather than a balmy sphere of oceans and continents.

Additional planets beyond 10 AU have been predicted by many investigators, although their detection will require further decades of observation (Rivera & Haghighipour 2007, Fischer et al. 2008, Raymond et al. 2008a). Fischer and colleagues fully expect “additional objects with masses ranging from Neptune to Saturn mass” in the outer system (Fischer et al. 2008), while ice dwarf planets resembling Pluto and Eris seem likely around any star where solids have accreted into larger bodies (Kenyon & Bromley 2009).

Last update March 2009